Improvements in or relating to Polymeric Compositions
The present invention concerns polymeric compositions and more specifically such compositions which contain a particulate filler or fillers.
Polymeric compositions incorporating hydrated alumina fillers, especially of the formula Al.sub.2 O.sub.3.xH.sub.2 O wherein x is 0.5 to 3, e.g. to impart flame retardent properties or to enhance the anti-tracking properties in electrical applications, frequently exhibit a tend a tendency to absorb water particularly when the composition comprises a polar polymer.
This tendency to absorb water is often directly or indirectly attributable to the particulate filler, i.e. to the hygroscopic nature of the filler per se or of the filler after incorporation into the polymeric composition. In the latter respect, it has been found that hydrated aluminas when incorporated into certain polymeric compositions can undergo, at least to a limited extent, a chemical or physical transformation yielding a chemical species substantially more hygroscopic than the incorporated filler per se.
The presence of such chemical species in a filled polymeric system may result in a very high water pick-up exposed to wet environments, especially hot wet environments.
The tendency of polymeric compositions to absorb water may render the use thereof in certain applications unsuitable, particularly in electrical insulation applications and especially flame retarded systems wherein reliance is placed on hydrated alumina fillers to provide flame retardancy.
The present invention is concerned with reducing or eliminating the tendency of hydrated alumina filled polymeric compositions to absorb water, hereinafter referred to as "water stabilization" and the expression "water stabilizer" is to be construed accordingly.
A first aspect of the present invention provides a water stabilized polymeric composition incorporating a hydrated alumina filler which is characterized by the presence of an effective amount of an antimony V compound.
A second aspect of the present invention provides water stabilized hydrated alumina characterized in that it includes an effective amount of an antimony V compound.
A third aspect of the present invention provides electrical equipment, for example a wire or cable, or a high voltage insulator, incorporating as electrical insulation a polymeric composition in accordance with the first aspect of the invention.
A fourth aspect of the present invention provides a water stabilized dimensionally recoverable, and especially heat-recoverable, article comprising a polymeric composition in accordance with the first aspect of the invention, preferably in cross-linked form.
A fifth aspect of the present invention provides a water stabilized adhesive composition or sealant composition comprising a polymeric composition in accordance with the first aspect of the present invention.
A sixth aspect of the present invention provides water stabilized panels and cladding especially reinforced e.g. glass reinforced, panels and cladding, for use e.g. in the construction industry and passenger transit vehicle industry comprising a polymeric composition in accordance with the first aspect of the invention.
The particulate fillers that may be employed in the compositions of the present invention are those which have a tendency to absorb water at least after incorporation into the composition. Particularly important classes of fillers include hydrated aluminas used as inorganic flame retardants and electrical anti-tracking agents, especially halogen-free such fillers. Preferred fillers are hydrated aluminas of the formula Al.sub.2 O.sub.3.xH.sub.2 O wherein x is 0.5 to 3, for example alumina mono-hydrates e.g. Al.sub.2 O.sub.3.xH.sub.2 O, but especially alumina trihydrate Al.sub.2 O.sub.3.xH.sub.2 O. In this specification the term hydrated alumina is also intended to include materials which are known as aluminum hydroxides e.g. Al(OH).sub.3 and aluminium oxyhydroxides e.g. AlO.OH. Other suitable hydrated alumina fillers include ammonium and sodium dawsonite. Mixtures of hydrated alumina fillers may be used if desired.
The hydrated alumina filler used is preferably selected from those having a surface area of at least 0.1 M.sup.2 /g, up to 300 mm.sup.2 /g and most preferably having a surface area of from 0.1 m.sup.2 /g to 100 m.sup.2 /g, particularly 2 to 40 m.sup.2 /g, and especially 4 to 16 m.sup.2 /g, as measured by the B.E.T. method. Outstanding improvements in the electrical properties of filled polymer compositions containing high surface area alumina trihydrate fillers may be achieved using the present invention.
The amount of filler incorporated in the compositions of the invention will naturally depend inter alia on the nature thereof although in general, amounts in the range 5 to 80%, more preferably 20 to 70%, especially 30 to 70%, e.g. 40 to 70% based on the weight of the composition, are preferred.
Whilst we believe that insolubilization of residual sodium in the filler plays a role in water stabilization by antimony V compounds, the complete mechanism of water stabilization is not fully understood and will vary depending on the nature of the insolubilizing agent. It is conjectured that in addition to forming a water insoluble compound with any available sodium present, the antimony V compound may also function as a water stabilizer by forming a water barrier around the filler particles thereby inhibiting water access to any hydroscopic moieties in the hydrated alumina even when the antimony V compound is not pre-coated onto the filler. Furthermore, and depending on the nature of the filler and antimony V compound, we believe that such water barrier may be the result of complex formulation between the antimony V compound and the surface of the filler particles.
Although not essential, it is preferred that the level of sodium impurity in the hydrated alumina should be reduced to a minimum. This may be achieved by pre-treatment, for example, washing with water or an aqueous medium particularly aqueous inorganic or organic acids, e.g. to a total sodium level less than 1000 ppm, more preferably less than 600 ppm of sodium measured as Na.sub.2 O, based on the weight of the hydrated alumina.
Reduction of the sodium impurity level may also be achieved by pre-treatment of the filler with a sodium complexing or chelating agent such as particularly zinc or magnesium uranyl acetate, but also uranic acid, molybdic acid, orthophosphoric acid, hypophosphorous acid, dodecamolybdophosphoric acid and polydentate ligands such as polyketonates. Such treatment is normally followed by washing of the filler to remove any water soluble compounds thus formed.
Many conventional polymer processing methods involve milling, internal mixing e.g. in a Banbury mixer, or other treatment stages likely to cause some break-up of the filler particules thereby exposing sodium ions previously trapped within the crystal lattice. It will be appreciated therefore that for many purposes merely reducing the soluble sodium content of the filler prior to blending with the polymer is not sufficient to water stabilize the filled polymer system.
The antimony V compound may be incorporated by distribution thereof throughout the composition or alternatively the particulate filler may be reacted by coating therewith as appropriate.
However, it has been found that greatly improved results are obtained if the hydrated alumina, an effective amount of the antimony V compound and the polymer, are processed together, for example on a mill or in an internal mixer, for an extended period of time. such processing is preferably carried out for at least 5 minutes, and preferably from 10 to 60 minutes. Alternatively, the hydrated alumina may be treated with the antimony V compound prior to blending with the polymer. Suitable antimony V compounds include, for example, the hydrated forms of antimony V oxide, especially the monohydrates Sb.sub.2 O.sub.5.H.sub.2 O, antimony alkoxides, for example antimony V pentamethoxide, and antimony pentahalides, for example antimony pentachloride. The treatment step may require to be followed by a washing treatment with an aqueous or organic solvent depending upon the antimony V compound used but at least in the case of the hydrated antimony V oxides this is not essential.
Whatever the mechanism of the water stabilization by the antimony V compound, it has been found in practice that further improved results may be obtained when the hydrated alumina and/or the antimony V compound is coated with a reagent and preferably one which will chemically react or physically bond with the surface of the hydrated alumina and/or antimony V compound.
As examples of reagents which may be employed to coat the surface of the filler and/or the antimony V compound there may be mentioned acids, both organic and inorganic, and reactive functional acid derivatives, e.g. esters, acid halides and anhydrides, and coupling agents such as titanates and silanes, and complexing or chelating agents including polydentate ligands such as B-polyketonates and Schiff base(imine) derivatives thereof.
Examples of organic acids include mono- or polybasic substituted or unsubstituted carboxylic, thiocarboxylic or sulphonic acids such as monobasic C.sub.2 -C.sub.20 acids, e.g. acrylic, acetic, n-propionic, n-butanoic, n-hexanoic, n-octanoic, lauric and stearic acids, aromatic acids, e.g. benzoic acid and polybasic C.sub.2 -C.sub.20 acids, e.g. oxalic, maleic, fumaric, malonic, succinic and itaconic acids and derivates such as the anhydrides and monesters, especially mono-alkyl or a-alkenyl esters, thereof.
Examples of substituted mono- or polybasic carboxylic or thiocarboxylic acids are amino acids, e.g. glycine, alanine and phenylalanine and hydroxy, particularly a-hydroxy, acids, such as lactic acid, salicylic acid, tannic acid and citric acid, especially citric acid.
Examples of inorganic acids include dilute mineral acids such as hydrochloric acid as well as phosphoric acid, molybdic acid, hypophosphorous acid and telluric acid.
Examples of coupling agents include organo-silicon and titanium derivatives such as silanes and titanates.
Examples of appropriate silanes are disclosed in UK Patent No. 1,284,082 and "Silane coupling Agents", Additives for Plastics, Vol. 1 pp 123-167, Academic Press 1978, and appropriate titanium compounds in S. J. Monte and G. Sugerman, J. Elastomers and Plastics, Volume 8 (1976) pages 30-49 and in Bulletins KR 0376-4 and 0278-7"Ken React Titanate Coupling Agents for Filled Polymers" published by Kenrich Petro Chem Inc., the disclosures of which are incorporated herein by reference.
Suitable polyketonates and Schiff base derivates include those derived from polyketones of formulae EQU R--CO--CH.sub.2 (CO--CH.sub.2).sub.m --CO--R.sup.1
Where R and R.sup.1 are the same or different and can be alkyl, alkenyl, aryl or alkaryl groups and m - 0 or an integer.
Examples of such compounds include acetyl acetone, allyl aceto-acetonate, di-acetyl acetone, benzoly acetyl acetone, dibenzoly acetone, and benzoyl allyl aceto-acetonate. A suitable Schiff base derivative is: EQU [CH.sub.3 COCH.sub.2 COCH:C(CH.sub.3)NHCH.sub.2 ].sub.2
Furthermore, by appropriate choice of pre-coating reagent, the treatment may additionally serve to reinforce the composition, for example by employing unsaturated reagents, particularly unsaturated carboxylic or thiocarboxylic acids or functional derivatives thereof, such as acrylic, methacrylic, vinylacetic, allylacetic, fumaric, maleic or itaxonic acids, or unsaturated esters, e.g. allyl esters of polybasic, particuarly dibasic, acids, e.g. the mono-allyl ester of malonic, maleic or succinic acids, whereby the unsaturated function enables grafting of the reagents to the polymeric matrix of the composition. Alternatively, reinforcement may be imparted by use of coupling agents.
Under certain circumstances, various desirable properties may be optimized by the use of a plurality of precoating reagents. Thus a combination of acrylic and stearic acids permits a satisfactory balance of physical and water stabilization properties.
The amount of pre-coating reagent(s) employed preferably lies in the range 0.1 to 15 weight per cent, more preferably 1 to 10 weight per cent based on the weight of the filler.
In some cases processing aids such as stearic acid and stearates, e.g. calcium stearate, and aluminium soaps such as aluminium di-isopropoxy diversatate or polyoxo carboxylates, such as polyoxo aluminium stearate, may be particularly useful.
Examples of appropriate aluminum soaps and polyoxo aluminium carboxylates are given in U.K. Patent No. 825,878, the disclosures of which are incorporated herein by reference.
Of the possible antimony V compounds, including antimony V complexes, which may be employed in the composition of the present invention particuarly preferred are the hydrated forms of antimony V oxide, especially the monohydrate i.e. Sb.sub.2 O.sub.5.H.sub.2 O, and precursors thereof. Such precursors include compounds which during processing or by other chemical or physical treatment may be converted into a hyrated antimony V oxide. However good results may also be obtained using antimony V alkoxides, for example, antimony V pentamethoxide.
The antimony V compound is present in an amount effective to improve the water stability of the composition, said amount being preferably in the range 0.1 to 60 weight per cent based on the composition, more preferably 0.1 to 25 weight per cent, although for water stabilization purposes only, it has been found that amounts in the range 0.1 to 10 weight per cent e.g. 0.5 to 10 weight per cent are quite satisfactory. However, it has also surprisingly been found that whilst lower amounts have no substantial positive effect on flame retardancy of the composition and indeed, in some instances a depression of flame retardance is observed, in higher amounts, e.g. above 5 weight per cent, preferably from 10 to 25 weight per cent, especially 10 to 15 weight per cent based on the composition, a substantial increase in flame retardance is observed at elevated temperatures, i.e. as measured by the British Ministry of Defense specification NES 715 (based on ASTM-D-2863) whilst still providing satisfactory water stabilization.
Polymeric compositions wherein the water stabilizers have been found to exhibit notable efficacy in reducing water absorption include non-cross-linked and cross-linked thermoplastic or elastomeric substituted or unsubstituted polyalkenes or alkene copolymers, such as alkene/alkene copolymers, vinyl ester homo- or copolymers and (meth)acrylate homo- or copolymers.
The term "copolymer" as used in herein is employed in a broad sense to mean any polymer containing at least 2 different monomeric species and includes terpolymers and the like.
Unsubstituted polyalkenes and alkene copolymers of interest include polyethylenes and alkene/alkene copolymers such as ethylene/alkene copolymers, especiality those disclosed in UK Patent Application No. 2019412A and Canadian Patent No. 873,828 the disclosures of which are incorporated herein by reference and elastomers such as EPDM (ethylene/propylene/diene monomer) and SBR (styrene/butadiene rubber).
The vinyl ester homo- or copolymers of interest are those derived from vinyl ester monomers of formula I ##STR1## wherein R.sub.1, R.sub.2, and R.sub.3 are each, independently hydrogen or a substituted or unsubstituted hydrocarbon group having up to 20 carbon atoms, e.g. C.sub.1 -C.sub.8 alkyl,
and R.sub.4 is a substituted or unsubstituted hydrocarbon group having up to 20 carbon atoms, which may optionally include one or more hetero atoms,
especially where R.sub.1, R.sub.2, and R.sub.3 are each independently hydrogen or C.sub.1 -C.sub.4 alkyl, and one of R.sub.1, R.sub.2, and R.sub.3 may also signify phenyl or benzyl, and R.sub.4 is an alkyl, alkenyl, alkoxyalkyl or alkoxy-alkenyl, aryl or aralkyl group having up to 20 carbon atoms e.g. phenyl, naphthyl or benzyl, or C.sub.1 -C.sub.4, alkyl- or alkoxyphenyl or benzyl, and particuarly when R.sub.4 is C.sub.1 -C.sub.15 alkyl, phenyl or benzyl.
Examples of specific vinyl ester monomers useful in the polymers of interest are vinyl acetate, vinyl propionate, vinyl hexanoate, vinyl octanoate, vinyl versatate, vinyl stearate, vinyl laurate, vinyl methoxy-acetate, vinyl trimethylacetate, vinyl isobutyrate, vinyl tert, pentoate, vinyl lactate, vinyl caprylate, vinyl pelargonate, vinyl myristate, vinyl oleate, vinyl linoleate, vinyl benzoate, vinyl (C.sub.1 -C.sub.4) alkoxy benzoate, vinyl octylphthalate, vinyl B-phenyl butyrate, vinyl B-naphthoate, vinyl ethyl phthlate and vinyl phenyl acetate.
Vinyl ester copolymers of interest include mixed vinyl ester copolymers e.g. copolymers of vinyl acetate with higher vinyl esters, e.g. vinyl laurate as well as copolymers with comonomers other than vinyl esters. Other such comonomers include unsaturated hydrocarbons such as olefins e.g., ethylene, propylene, particuarly C.sub.4 -C.sub.12 a-unsaturated olefins, e.g. but- 1 ene, hex-1-ene and oct-1 -ene, styrene and unsubstituted or substituted esters, e.g. C.sub.1 -C.sub.12 (meth)acrylates and other vinyl monomers, e.g. vinyl chloride.
When copolymers with comonomers other than vinyl esters are employed, then preferably the vinyl ester monomer(s) is present in at least 5 mole %, preferably at least 10 mole %, e.g. 20-95 mole %, more preferably at least 30 mole %, e.g. 40-95 mole %.
The (meth)acrylate homo- or copolymers of interest are those derived from (meth)acrylate monomers of formula II, ##STR2## wherein R.sub.5 is H or CH.sub.3
and R.sub.6 is a substituted or unsubstituted hydrocarbon group having up to 20 carbon atoms, which may optionally include one or more hetero atoms, preferably an alkyl, alkenyl, alkoxyalkyl or alkoxyalkenyl aryl or aralkyl group having up to 20 carbon atoms, e.g. phenyl, naphthyl or benzyl, and particuarly C.sub.1 -C.sub.15 alkyl, phenyl or benzyl.
Preferred homo polymers include polyethyl acrylate, polybutyl acrylate. Particularly preferred (meth)acrylate copolymers are ethylene/ethyl acrylate (18 weight per cent), ethylene/ethyl methacrylate (20 weight per cent) and terpolymers of ethylene, methyl acrylate and a cure-site carboxyl-containing monomer such as the terpolymer commercially available from DuPont under the trade name "Vamac" and analogous polymers described in UK Patent No. 1,548,232 the disclosure of which is incorporated herein by reference. Other copolymers suitable for use in the present invention include those derived from any of the above mentioned monomers and a cure site monomer such as 2-chloroethyl vinyl ether or 2-chloroethyl acrylate.
Other polymers of interest which may be employed in the compositions of the invention include epoxy resins especially cycloaliphatic epoxy resins such as those employed for high voltage insulation applications, e.g. Araldite CY175 and Bakelite FRC 4221, especially when cured with cycloaliphatic acid anhydrides e.g. hexa-hydro phthallic anhydridie, unsaturated polyesters especially glass filled polyesters, and polyamides especially aliphatic polyamides such as nylons.
Furthermore, such polymers may be employed as a blend system containing other polymers. Preferred polymer blends are those comprising vinyl ester homo- or copolymers, (meth)acrylate homo- or copolymers, and/or substituted or unsubstituted polyalkenes or alkene copolymers. Blend polymers of interest are those which are preferably compatible, either alone or in association with compatibilizing agents, with the aforedescribed polymers and which do not render the compositions unsuitable for the purpose for which they are intended. Such blends may include blends of two or more of the aforedescribed polymers and which do not render the compositions unsuitable for the purpose for which they are intended. Such blends may include blends of two or more of the aforedescribed polymers or blends with other polymers including thermoplastic and elastomeric polymers, examples of which are polyesters and polyethers including segmented polyether ester copolymers of the type available from DuPont under the trade name Hytrel and described in Polymer Engineering and Science 14 volume 12, 848-852 1974 "Segmented Polyether Ester copolymers--a new generation of high performance elastomers", silicone resins and elastomers and acrylic elastomers. Some preferred polymer blends are described and claimed in UK Patent No. 1,284,082 and in German Offenlegenschrift No. 2815520 the disclosures of which are incorporated herein by reference.
In the case of vinyl ester or (meth)acrylate polymers, when blends are employed such polymers are preferably present in the blend in at least 20 weight per cent, e.g. 30-99 weight per cent, more preferably at least 40 weight per cent, e.g. 50-99 weight per cent.
The polymer composition may also contain other additives such as organic halogen containing or inorganic flame retardants, or organo-phosphorous compounds, anti-tracking agents, high voltage erosion suppressants, stabilizers, e.g. antioxidants and ultra-violet stabilizers, fillers, pigments, colourants and the like. In general it is preferred that the composition contain little or no halogen e.g. less than 15 weight per cent based on the composition, more preferably less than 10 weight per cent, especially less than 5 weight per cent of halogen atoms.
For most purposes, it is preferred that the compositions of the invention are substantially cross-linked.
The degree of cross-linking of the compositions may be expressed in terms of the gel content (ANSI/ASTM D2765-68) of the cross-linked polymeric composition, i.e. excluding non-polymeric additives that may be present. Preferably the gel content of the cross-linked composition is at least 10% more preferably at least 20%, e.g. at least 30%, more preferably at least 40%.
The compositions of the invention are produced in conventional manner, for example, by blending together the components of the composition in a Banbury mixer.
They may then be processed into shaped articles, for example, by extrusion or molding, and when desired simultaneously or subsequently cross-linked. Shaped articles so produced also form part of the present invention.
When desired the polymeric components of the composition may be cross-linked either by the incorporation of a crosslinking agent or by exposure to high energy radiation. Examples of suitable cross-linking agents are free radical initiators such as peroxides for example, dicumyl peroxide, 2,5-bis-(t-buty peroxy)-2,5-dimethylhexane, 2,5-bis-(t-butyl-peroxy)-2, 5-dimethyl-hexhyne-3, and a,a-bis(t-butyl-peroxy)-di-iso propylbenzene. Other examples of appropriate cross-linking agents are disclosed in C. S. Sheppard & V. R. Kamath Polymer Engineering & Science 19 no. 9 597-606, 1979 "The Selection and Use of Free Radical Initiators" the disclosures of which is incorporated herein by reference. In a typical chemically cross-linked composition there will be about 0.5 to 5 weight per cent of peroxide based on the weight of the polymeric composition. The cross-linking agent may be employed alone or in association with a cocuring agent such as a poly-functional vinyl or allyl compound, e.g. triallyl cyanurate, triallyl isocyanurate or pentaerythritol tetra methacrylate.
Radiation cross-linking may be effected by exposure to high energy irradiation such as an electron beam or x-rays. Radiation dosages in the range 2 to 80 Mrads, preferably 2 to 50 Mrads, e.g. 2 to 20 Mrads and particuarly 4 to 15 Mrads are in general appropriate.
For the purposes of promoting cross-linking during irradiation preferably from 0.2 to 5 weight per cent of a prorad such as poly-functional vinyl or allyl compound, for example, triallyl cyanurate, triallyl isocyanurate or pentaerythritol tetramethacrylate are incorporated into the composition prior to irradiation.
The compositions of the invention are particuarly suitable for electrical insulation use, e.g. where flammability requirements are stringent or high voltage anti-tracking properties are desired and water absorption highly undesirable, for example, wire and cable applications in confined areas such as aircraft, ships, mines or railways including metro systems, or termination and splicing of high voltage cables or as insulation in non-tracking high voltage applications, e.g. insulators, bushings and bus bar systems.
The production of electrical insulation material may be achieved by conventional techniques, for example, by extrusion of the non-cross-linked composition, as an insulator, onto the electrical equipment such as a copper conductor as a primary insulation, or a bundle of primary insulated copper wires as a cable jacket and preferably, simultaneously or subsequently cross-linking the applied insulation.
As aforedescribed, the compositions of the present invention are also particuarly suitable in cross-linked form for the production of dimensionally recoverable articles, that is to say, articles the dimensional configuration of which may be made substantially to change when subjected to an appropriate treatment. Of particular interest are heat-recoverable articlads, the dimensional configuration of which may be made substantially to change when subjected to an appropriate treatment. Of particular interest are heat-recoverable articles, the dimensional configuration of which may be made substantially to change when subjected to heat treatment. Heat-recoverable articles may be produced by deforming a dimensionally heat-stable configuration of the article to a dimensionally heat-unstable configuration, the article assuming or tending to assume the heat-stable configuration thereof on the application of heat alone. As is made clear in U.S. Pat. No. 2,027,962, however, the original dimensionally heat-stable configuration may be a transient form in a continuous process in which, for example, an extruded tube is expanded, whilst hot, to a dimensionally heat-unstable form.
Alternatively a preformed dimensionally heat-stable article may be deformed to a dimensionally heat-unstable form in a separate stage. In the production of dimensionally recoverable articles, the composition may be crosslinked at any stage in the production process that will accomplish the desired dimensional recoverability. One manner of producing a heat-recoverable article comprises shaping the pre-cross-linked composition into the desired heat-stable form, subsequently cross-linking the composition, heating the article to a temperature above the crystalline melting point or, for amorphous materials the softening point, as the case may be, of the polymer, deforming the article and cooling the article whilst in the deformed state so that the deformed state of the article is retained. In use, since the deformed state of the article is heat-unstable, application of heat will cause the article to assume its original heat-stable shape sleeves for covering and/or sealing splices and terminations in electrical conductors, for environmentally sealing damaged regions or joints in utility supply systems, e.g. gas or water pipes, district heating systems, ventilation and heating ducts and conduits or pipes carrying domestic or industrial effluent.
Adhesives and sealants in accordance with the fourth aspect of the invention are particuarly those which find use in electrical applications for example in harnessing systems, and especially in high voltage applications wherein anti-tracking fillers such as hydrated alumina are employed.
Preferred sealants include elastomer/tackifier formulations such as butyl rubber/polyisobutylene or epichlorophydrin rubber/polyketone resin compositions. Preferred adhesives include epoxy, especially cyclo aliphatic epoxy and silicone based adhesives.
Paneling and cladding materials in accordance with the fifth aspect of the invention include materials such as those disclosed and claimed in UK Patent Applications Nos. 2035333A and 2044777A the disclosures of which are incorporated herein by reference, as well as epoxy moulding compositions and unsaturated polyesters particuarly when glass-fibre reinforced. Such materials may be processed into final shape by thermoforming, e.g. by vacuum forming, moulding or laying-up techniques.
The invention is illustrated by the accompanying examples wherein parts and percentages are by weight and temperatures are expressed in .degree. C.